The present invention relates to a die casting machine for manufacturing heat resistant impellers.
A heat resistant impeller, such as an exhaust gas turbine rotor for a supercharger used in combination with an internal combustion engine comprises, as shown in FIGS. 13A and 13B, a boss m and a number of blades n radially projecting from the boss, each of the blades having considerably curved surfaces. Heretofore, such impellers have been manufactured mainly by the lost wax process requiring relatively complex dies. Since such dies cannot be manufactured simply and without consuming substantial time, cost of the product could hardly be reduced. Accordingly, application of metal mold casting or the die cast process has been considered to be superior. The turbine rotor in general has to withstand hot exhaust gas whose temperature amounts to 800.degree. C. Thus, the rotor usually consists of Ni-, Co- or Fe-based metals having a high melting temperature of about 1,400.degree. C., for example. Since the metal mold is subjected to molten metal of such a high temperature, long life of the mold can never be expected. A die cast machine encounters further problems relating to complexity in the shaping of die, arrangement of drive means to open or close the die, as well as the combination of the die and the injection means. Those problems make it difficult to provide commercially applicable die cast machines for such particular purposes.
Turbine rotors or other mechanical components whose rotational speed is high, should be symmetrical with respect to the rotational axis not only in the shape but in the texture of the structure of the material as well. To this end, preferably, molten metal at the time of casting is fed into the die cavity rapidly and symmetrically with respect to the rotational axis, and after completion of casting the molten metal is caused to solidify uniformly and symmetrically with respect to the axis. Otherwise, blowholes are formed within the product, and distribution of the texture of the structure, i.e., of the specific weight and strength, becomes asymmetrical with respect to the rotational axis. With these defects, products capable of withstanding high revolutional speed cannot be obtained.
Referring to FIG. 1, a known die casting machine shown therein for manufacturing impellers by die casting aluminum or ordinary steel comprises: a die assembly including a stationary die a and a movable die b; and an injection assembly including a runner c extending from a chamber e having a gate d, and a plunger f for injecting the molten metal within the chamber e through the runner c into the cavity h formed by the dies a and b. The injection is effected asymmetrically with respect to the axis of the dies or of the product. This known machine encounters the following problems. Firstly, since the distance between the gate d and the cavity h is relatively long, the temperature of the molten metal is liable to be dissipated and reduced while flowing through the runner. In order to maintain its temperature without a substantial decrease, molten metal should be delivered to the die assembly at a relatively high speed. Secondly, however, under such a high speed flow condition, flow of the molten metal would be turbulent by which the dies would get damaged prematurely. Lastly, in order to die cast metals whose melting temperature amounts to 1,400.degree. C., for example, the temperature of the molten metal should be as high as 1,500.degree. C. In contrast to this, even though the dies consist of tungsten or molybdenum alloys, the die should not be heated to more than 300.degree. C. in view of its life. Thus, the difference in temperature between the die and the molten metal amounts to 1,200.degree. C. by which the metal is not cooled and solidified uniformly, and the texture of the texture will not be uniform.